1. Field of the Invention
The present invention relates generally to a method of synchronizing with an uplink channel and a method of determining a propagation delay in a wireless communications system.
2. Description of the Related Art
A cellular communications network typically includes a variety of communication nodes coupled by wireless or wired connections and accessed through different types of communications channels. Each of the communication nodes includes a respective protocol stack that processes the data respectively transmitted and received over the communications channels. Depending on the type of communications system, the operation and configuration of the various communication nodes can differ and are often referred to by different names. Such communications systems include, for example, a Code Division Multiple Access 2000 (CDMA2000) system and Universal Mobile Telecommunications System (UMTS).
UMTS is a wireless data communication and telephony standard which describes a set of protocol standards. UMTS sets forth the protocol standards for the transmission of voice and data between a base station (BS) or Node B and a mobile or user equipment (UE). UMTS systems typically include multiple radio network controllers (RNCs). The RNC in UMTS networks provides functions equivalent to the base station controller (BSC) functions in GSM/GPRS networks. However, RNCs may have further capabilities including, for example, autonomously managing handovers without involving mobile switching centers (MSCs) and serving general packet radio service (GPRS) support nodes (SGSNs). The Node B is responsible for air interface processing and some radio resource management functions. The Node B in UMTS networks provides functions equivalent to the base transceiver station (BTS) in GSM/GPRS networks. Node Bs are typically physically co-located with an existing GSM base transceiver station (BTS) to reduce the cost of UMTS implementation and minimize planning consent restrictions.
FIG. 1 illustrates a conventional communication system 100 operating in accordance with UMTS protocols. Referring to FIG. 1, the communication system 100 may include a number of Node Bs such as Node Bs 120, 122 and 124, each serving the communication needs of UEs such as UEs 105 and 110 in their respective coverage area. The Node Bs are connected to an RNC such as RNCs 130 and 132, and the RNCs are connected to a MSC/SGSN 140. The RNC handles certain call and data handling functions, such as, as discussed above, autonomously managing handovers without involving MSCs and SGSNs. The MSC/SGSN 140 handles routing calls and/or data to other elements (e.g., RNCs 130/132 and Node Bs 120/122/124) in the network or to an external network. Further illustrated in FIG. 1 are conventional interfaces Uu, Iub, Iur and Iu between these elements.
The conventional communication system 100 may employ two mechanisms for mobility between Node Bs 120/122/124; namely, a soft handover approach and a hard handover approach. In soft handoff, multiple legs or radio channels (e.g., communication links between a mobile station and multiple Node Bs) are used to transfer digital messages from the communication system 100 to the UE (e.g., UE 105). The UE 105 combines the analog data received from the multiple legs into a single analog waveform before attempting to decode the message. Conversely, when transmitting to the communication system 100, the UE 105 transmits in soft-handover mode to multiple base stations, each of which decodes the message independently and forwards the decoded message to an RNC (e.g., RNC 130). The RNC 130 selects one of the forwarded messages as being representative of the data message sent by the UE 105.
Legs are added to a UE 105's active set whenever the pilot strength of a particular base station is received by the UE 105 with a signal-to-noise ratio (SNR) above a threshold level. A leg may be added to the active set even though the channel itself may not yet be strong enough to support data transmission. However, time consuming synchronization procedures are performed before the channel may be used for data transmission. Namely, in soft handoff, timings between each of the base stations in the active set are synchronized with each other. Such synchronization introduces transmission latency.
More recently, a base station router (BSR) or integrated base station has been adopted. A BSR collapses, among other functions, the functionality of a RNC and a base station (or Node B) into a single processing entity, thereby reducing latency (e.g., because the Iu interface is eliminated). BSRs are, for example, described in U.S. patent application Ser. Nos. 11,094,436 and 11,094,430, each filed on Mar. 31, 2005. Since latency is greatly reduced with the BSR structure, BSRs often employ hard handoff as opposed to the above described soft handoff approach.
The hard-handover procedure is a break-before-make mobility procedure in terms of the radio channel used by the UE. In hard handoff, as a mobile station or UE moves throughout the communication system 100 and within the serving areas of different Node Bs, only one connection between the UE and a respective Node B is active at any particular time because a current channel is discarded before a new channel is established. However, radio outages occur during the time period starting from when the current channel is dropped until the new channel is established.
In the communication system 100, a hard-handover procedure typically involves sending a layer-3 control message to a UE indicating the channel parameters for the new channel. The layer-3 control message may contain one or more of a downlink channelization codes, a downlink scrambling code, an uplink channelization code and an uplink scrambling code. When the UE (e.g., UE 105) receives the layer-3 control message, the UE 105 ceases communication on the old channel and begins searching for a new downlink channel associated with the new Node B (e.g., Node B 122). Once the UE 105 finds the new downlink channel, the UE 105 transmits to the new Node B 122 in accordance with the channelization parameters indicated by the layer-3 control message.
Within the UMTS standard, the synchronization procedure for the UE to align itself with a new downlink is performed by de-referencing the chip offset from the base station's pilot channel. The layer-3 control message to the UE indicates the chip offset of the new radio channel in terms of chips to the new base station's pilot channel. Each chip represents a portion of an analog wave form, and a UMTS may use, for example, 3.84 million chips per second (Mcps). The communication system 100 thereby informs the mobile or UE at which offset it should look or search for the new downlink. Using this offset information, the mobile is able to more quickly (e.g., within 40 ms) lock onto the new downlink if the new channel can be de-referenced to a known pilot channel, which has already been measured by the mobile. Once locked onto the new downlink, the UE is said to be synchronized.
In UMTS, the uplink chip offset is related to the above-described downlink chip offset. Generally, each radio channel is divided into fixed length time intervals; for example, a Transmission Time Interval (TTI) having 15 slots each with 2560 chips. Each TTI may have a fixed duration (e.g., 10 milliseconds (ms)). An uplink channel's TTI starts 1024 chips after reception by the mobile of a downlink channel's TTI. Additionally, the uplink TTI does not begin until the downlink is synchronized by the mobile. Thus, the uplink TTI begins after the downlink is synchronized.
The base station or Node B analyzes or “searches” uplink transmissions sent by the UE so as to synchronize with the uplink. The propagation delay between the new base station or Node B and the mobile or UE, or vice versa, is typically not known. Before the base station attempts to decode the messages transmitted from the mobile, the base station searches through the received uplink chip space until an uplink pilot and a TTI boundary has been found.
The chip search ranges can be substantial in UMTS systems, and base stations in UMTS systems typically do not include processors fast enough to perform real-time searching. In other words, larger chip search spaces generate a larger lag, which increases the processing time required for the Node B to synchronize with the uplink TTI, and increases radio outage lengths.
Also, the mobile is not informed of when the base station is synchronized with the uplink TTI. Rather, the mobile simply begins transmitting data as soon as it is able without taking the base station's synchronization into account. Thus, data packets transmitted by the mobile may be lost if they arrive at the base station before the base station is synchronized. Transmitted data packets, which are lost during the above-described search procedure, are then retransmitted at a higher protocol layer. This retransmission of lost data packets may cause latency effects in the uplink (e.g., between 120 to 340 ms).